Amid talk that former Massachusetts Sen. Scott Brown plans to form an exploratory committee for a New Hampshire Senate run, conservative group American Crossroads is launching a $650,000 ad campaign against Sen. Jeanne Shaheen, the Democrat who holds the seat.

The ad campaign, which will be broadcast statewide throughout next week, aims to give Mr. Brown a bump in the polls as he prepares to announce his candidacy. A Suffolk University-Boston Herald poll last week showed Mr. Brown trailing Ms. Shaheen by 13 points, and found his favorability ratings 20 percentage points lower than hers. Still, as recently as January, two polls showed the race would be neck-in-neck if Mr. Brown ran.

Ms. Shaheen, who has seen her ratings sag since she was first elected to the Senate in 2008, is considered vulnerable due to her support of the Affordable Care Act, which has brought big problems to the Granite State. Republicans have been imploring Mr. Brown to join the race, as polls show that among possible GOP contenders, he is most likely to win the seat. Republicans hope his candidacy will put New Hampshire in play, just as GOP Rep. Cory Gardner’s Senate bid did in Colorado earlier this month.

Democrats groups including the Senate Majority PAC and the League of Conservation Voters have already run ads attacking Mr. Brown, and American Bridge sent out an email Friday morning highlighting gaffes from his 2012 Senate campaign in Massachusetts, which he lost to Sen. Elizabeth Warren.

American Crossroads’ campaign is its second buy of the cycle. In the weeks leading up to Tuesday’s special election in Florida’s 13th district, the group spent hundreds of thousands of dollars on an ad campaign knocking Democrat Alex Sink. Republican David Jolly won the race by a slim margin.

Mr. Brown is expected to announce his exploratory committee Friday afternoon at the Northeast Republican Leadership Conference in Nashua, N.H.

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